By training on a crowded mat, we improve awareness and learn to judge distances precisely when we throw.

Also, for techniques that require more room, like sacrifice throws, and for stand-up randori, we can't have everyone on the mat at the same time.

When we have a larger class, like the one pictured, I often split the class in two: one half of the class practices restraint & control techniques off the mat, while the other half gets more room on the mat for throwing - and then we swap over.

Funnily enough I went on a jujitsu/kobudo seminar on Saturday at which there were about 140 people attending. Initially I thought, 'no-way!'However we were divided into groups of about 16-20 and rotated around about 8 matted areas. It was still a little crowded on the mats but it worked fine. You're right, you just learn to have more awareness and be more careful. It was a great course - have you heard of Bartitsu by the way? Very interesting that one!